1. Field of the Invention
The present invention relates to an organic light emitting device that uses an organic light emitting diode (“OLED”) and a method of manufacturing the device, and more particularly, to an active matrix organic light emitting device having a pixel circuit that drives the OLED using an organic thin film transistor in each pixel and a method of manufacturing the device.
2. Description of the Related Art
An organic light emitting diode (“OLED”) is a light emitting device that generates light using an organic electroluminescence phenomenon, where light having a particular wavelength is generated by energy of excitons formed by recombining electrons injected from a cathode and holes injected from an anode, in a low molecular weight organic thin film or a polymer organic thin film.
Display devices that use the OLED can be divided into passive matrix (“PM”) types and active matrix (“AM”) types according to the driving method. The AM type organic light emitting device includes a pixel circuit that drives each pixel, arranged in a matrix with the pixels. The pixel circuit is switched by a thin film transistor (“TFT”), and stores pixel related information. The AM type has the advantages of superior contrast, less image flickering, and quicker response time over the PM type, since the AM type can be driven at a higher speed than the PM type.
FIG. 1 is a cross-sectional view illustrating a conventional AM type organic light emitting device. To facilitate understanding of the structure, the cross-section of a unit pixel that includes a switching TFT and an OLED is depicted. In FIG. 1, a TFT 20 structure is included on one side of a substrate 10, and an OLED 30 structure is included adjacent to the TFT 20 structure on the substrate 10. This type of structure is employed by an organic light emitting device disclosed in U.S. Pat. No. 6,784,032. The TFT 20 structure includes a gate electrode 21, a gate insulating layer 22, a source electrode 24, a drain electrode 23, and a semiconductor unit 25. The OLED 30 structure includes a pixel electrode 31 connected to the drain electrode 23, a bank unit 32 that defines a light emission region, and an organic light emitting layer 33 and a transparent electrode 34 sequentially stacked in the light emission region. A protection film 40 is also employed.
The conventional AM organic light emitting device has a simple structure and is easy to manufacture, but has a low aperture ratio, which is the ratio of the area occupied by a light emission region with respect to the unit pixel area.
FIG. 2 is a cross-sectional view illustrating a conventional top-surface emitting organic light emitting device. In FIG. 2, to facilitate understanding of the structure, the cross-section of a unit pixel that includes a switching TFT and an OLEDis depicted. In FIG. 2, a TFT 20 structure is included on a substrate 10, and an OLED 30 structure is located on the TFT 20 structure. An organic light emitting device having this type of structure is disclosed in U.S. Pat. No. 6,538,390. A planarization film 41 is interposed between the TFT 20 structure and the OLED 30 structure, and the drain electrode 23 of the TFT 20 structure is connected to the pixel electrode 31 of the OLED 30 structure through an interlayer connection unit 29. A protection film 40 is also employed.
The conventional top-surface emitting organic light emitting device has a structurally high aperture ratio, because most of the pixel region can be used as the light emission region. However, the organic light emitting device has a high possibility of contamination with moisture, water vapor, and oxygen from the air, since the organic light emitting device is close to the external air. Thus, the protection film 40 must be provided with a sufficient thickness, thus increasing a thickness and overall size of the device.
Also, when an organic thin film transistor (“OTFT”) is used as the TFT 20 structure in the top-surface emitting organic light emitting device, the OTFT may be degraded during the process for forming the OLED 30, since the process temperature of the OLED 30 is higher than the process temperature of the OTFT.